Four Geological Eras

The history of the Earth is categorized into four different geological time eras beginning with, the Precambrian Time (4.6 billion-544 million years ago), the Paleozoic Era (544-248 million years ago), the Mesozoic Era (248-65 million years ago), and the Cenozoic Era (65 million years ago to the present). Eras are broken down further into smaller units called periods, which help scientists detect changes in Earth's history. Geologists's main sources of evidence for geological eras, and periods are the ages of rocks. There are several methods geologists't use to determine the age of rocks such as, relative age, the law of superposition, index fossils, relative dating, and radioactive dating. Through scientific reasoning and evidence this research
However, in 1913 the first geologic time scale was presented by Arthur Holmes; this was deciphered after the discovery of radioactivity by Henri Becquerel in 1896. Holmes used radioactivity to deduce that the Precambrian Era was between 4.6 billion-544 million years ago; the first milestone is the formation of Earth, and the second milestone is the beginning of life. As science and technology progresses scientists have a more exact date, which is, that the Earth is 4.543 billion years old. Scientists formulated the Precambrian era by practicing radioactive dating: knowing the number of radioactive parent atoms present when a rock formed and the number present now. The number of parent atoms initially present is the number present now plus the number of daughter atoms formed by the decay, both of which are amounts that can be measured. Moreover, another method that was used to determine when Earth formed was by reporting the proportion of lead types located in meteorites and comparing them to lead proportions located in other rocks on the Earth and other meteorites. Clearly, using radioactive dating and the comparison of rocks was essential factors in answering the questions on Earth's
These one-celled organisms helped to make the air and water around the Earth full of oxygen, forming new life. Next, photosynthetic organisms became part of Earth by using carbon dioxide from the atmosphere, and releasing oxygen. The last life form to develop in this era were simple soft bodied animals, since soft bodies do not have vertebrates there are not many fossils as evidence from this era. Scientists know that one-celled organisms, photosynthetic organisms, and soft bodied animals were the first species on earth by the following information, “The amount of C-14 in any sample of carbon containing material can be found by measuring the level of radioactive decay, and comparing that with the decay rate observed in a carbon sample exposed to the continual mixing at the surface of the earth of C-12 and C-14 produced in the upper atmosphere. Using the ratio of C-14 to total carbon, one can determine the age of the sample.” As a culmination, carbon dating, examining index fossils, and using relative dating reveal to scientists the milestone of the first organisms in the Precambrian